While Gekko talking, the camera rounds around him to capture the sun itself.
Before Gekko starts talking, he gazes upon the sun for a few seconds to get the inspiration to give that part of the speech, then about 10 seconds later you will see the sun on the horizon.
The scene is supposedly on the beach where his house is and the ocean is to the east - this means it must be the sunset, hence the darkness over the ocean.
Unfortunately, unlike the daylight itself, sunrise and sunset are happening very fast in only a few minutes until the sun is fully up or disappears. With its red-ish color palette, these are the moments that turn the view into a beautiful painting if there is enough cloud around.
I think the scene was tried to be shot in one go, they captured the beauty of the sun during the talk but failed the same to get a good one over the ocean. and for the budget, they decided to go with it.
As for the talk itself, that part might even be improvised by the actor, director caught it and follow to a complete and added the scene after. Or it was part of the script and they probably missed the narrow window to get the shot and continued with the darker one. (I was wrong, check the supporting material below)
I used "I think" and "might be improvised" to mean it is my deduction, just like the other answer. The whole scene takes only about 2 minutes with only a few seconds for the OP's mentioned shot. If anyone finds an official documentary, enlighten us. Otherwise, every answer given will be deductions on our side.
like IMDb, there is a movie script database,
imdbs.com, and I have found one script for the movie (if you think the site is a scam, let us know)
In it, you will find that the beach house is in Bridgehampton and on the ocean side. You can also search for the same house as it was in the news lately.
The speech is in the script, so I was wrong about one of my assumptions.
The script is written for sunrise (dawn) and Gekko even has his morning robe as he has "been up all night". But we don't see the sun over the ocean; it is on the mountain over the houses which is the sunset at that location.
This is a production trick over the costs of reshoots. Only observant eyes will notice the difference. As long as we have the sun on the horizon, we can trick the audience to say it is sunrise, albeit it is not.